Gas turbine engines have pressure sensors for monitoring the pressure in the engine working gas at the various engine stages. The pressures are used, for example, for engine control and health monitoring.
A common form of sensor is a resonant cylinder sensor, in which the gas whose pressure is to be sensed is admitted into a cylinder-like structure having a resonant frequency. The internal pressure of the structure changes with the pressure of the sensed gas, producing a modulation in the resonant frequency which can be detected to determine the pressure. Such sensors can provide very stable readings over time.
As the gas is admitted directly into contact with the resonant structure, such sensors are generally provided with inlet filters to prevent the ingress of particulate matter.
The sensors may also have heating systems keeping them slightly hotter than ambient condition in order to discourage the formation of condensation which can affect the measurements through the presence of liquid and/or corrosion.
In aero engines, the sensors are typically located in benign environments (i.e. cool and low vibration) with tubes attached to their inlets which carry the pressure from pressure tappings at the engine locations where the pressure is to be measured. For example, the sensors can be placed within the Electronic Engine Controller (EEC).
However, a problem can arise when the working gas pressure increases with higher engine thrust. In such a situation, compressed air moves down the tube and, as it travels towards the sensor, cools down causing humidity in the air to condense within the tube. The condensation is not expelled when the engine thrust reduces and more condensate is captured every time the thrust is increased. A water trap can be located at the lowest point in the tube for the condensate to accumulate until emptied by a maintenance-fitter. However, even if the sensor is mounted with its gas inlet at its bottom to encourage condensate formed within the sensor to drain away from it, water and other liquid pollutants such as sulphuric acid can enter the sensor causing damage through corrosion and their presence can degrade the accuracy of readings.
Pressure sensors can be provided with flexible diaphragms to prevent water and pollutants from reaching the sensor. However, such diaphragms can only be used where limited changes in pressure are to be measured. They are unsuitable for use on aero engines, as the pressure changes may be from 1 to 30 atmospheres and the change in volume which they would need to accommodate is too large.